Try deworming as mentioned, but worst case if she hasn't layed in a while she might be an internal layer. I had a BJG who's bum swelled up but stayed soft and she lived like that for years afterwards, just never layed anymore...who knows why she was so round!
good point :/
Anyway she looks quite healthy besides the poop. I thinks worms are most likely the cause. You can also look through her manure and see if you can spot any worms in there. Usually only the worsed cases have worms in the manure but it's worth a try. You can also feed a little raw garlic as it is a natural wormer...just mix it into a little yogurt or oatmeal. Some apple cider vinigar couldn't hurt either